This specification relates to classifying documents.
The Internet provides access to a wide variety of resources, for example, documents, web pages, image files, video files, or audio files. A search system can identify resources responsive to queries submitted by users and provide information about the resources in a manner that is useful to the users. Responsive resources can be identified in part based on a classification of resources. The users can navigate through, e.g., select, search results to obtain information of interest.